The present invention is generally directed to fluid motors. More particularly, it is directed to a fluid motor which permits the utilization of spring type poppet cushions without the use of conventional cylinder spacers or stop tubes at each end of the cylinder. This permits a reduction in the length of such fluid motors to the extent that they may satisfy national interchangeable standards.
Heretofore, it has been common practice in the field of fluid motors to provide a cushion effect at the ends of the piston stroke for purposes of preventing destructive impact of fluid motor components and to reduce the noise of operation of the motor. Ordinarily, such cushioning is effected by sealing off the major portion of the exhaust flow immediately prior to the termination of the piston stroke. This enables fluid pressure to build up against the exhaust side of the piston and thereby provide a decelerating effect on the piston.
One prior art fluid motor cushion arrangement employs a poppet type cushion to effect the cushioning or deceleration of the piston. Ordinarily, the poppet type cushion includes a cushion sealing element which is slidably movable on the piston rod of the fluid motor. Normally, the cushion element is spring biased in a direction away from the piston. The cushion sealing element will sealingly engage with the inlet-exhaust opening formed in the end wall of the cylinder. Once such cushion element closes the opening, it normally serves to cut off the exhaust path of the fluid therethrough. Thus, a quantity of pressurized fluid, either air or hydraulic fluid, is trapped between the piston and the cylinder end wall. During continued movement of the piston to the end of its stroke, the trapped fluid is metered from the contracting chamber by means of a metering valve. Such metering of the trapped fluid slows the rate of piston movement to thereby provide for the desired cushioning effect at the end of the piston stroke.
Although poppet-type cushion devices function quite satisfactorily, they nevertheless require the use of spacers or stop tubes internally of the cylinder so as to prevent the crushing of the spring associated therewith as the piston moves towards the end of its stroke within the cylinder. Typically, these spacers are generally cylindrical tubular members which are positioned at opposite ends of the cylinder and ordinarily serve to contact the piston as it moves toward the end wall of the fluid cylinder. By virtue of the spacers, the piston movement will be stopped at a point spaced axially from the end wall. The axial space thereby provided is sufficient to retain the structure associated with the cushion, including the compressed spring. Thus, the piston will not crush or otherwise deform the spring associated with the cushion device.
With the above construction, however, the overall length of a fluid cylinder having poppet-type cushions is necessarily longer than the overall length of a non-cushion cylinder structure having the same piston stroke. More specifically, if a non-cushion cylinder structure has a piston stroke of predetermined dimension, the length of the cylinder housing must be increased by the total length of the two spacers in order to provide a cushion cylinder with the same piston stroke. If the length of the cylinder were not increased and the spacers simply inserted into the non-cushion cylinder housing, the stroke of the piston would be decreased by the length of the spacers and would not be operatively equivalent to the non-cushion structure. In the industry, there are national standards that define the external dimensions of the cylinder and there are requirements that both the non-cushion and cushion types of cylinder be interchangeable in that they both have the same external dimensions. These standards do not permit any increase in the cylinder length. These standards can be met by cushion structures which are fixed to opposite sides of the piston; but to applicant's knowledge, there are no commercially available poppet-type cushion structures which are interchangeable with non-poppet-type cylinders.